ELECTROLYTIC COMPOSITION BASED ON SULFONIC ACID COMPRISING A PHOSPHORUS ADDITIVE

Abstract

The present invention relates to an aqueous electrolyte composition comprising a sulfonic acid, optionally sulfuric acid, redox metal ions, and at least one inorganic additive (A) comprising at least one phosphorus atom whose degree of oxidation is less than or equal to +5. The present invention also relates to an electrochemical cell comprising said electrolyte composition and an oxidation-reduction battery (also called a redox battery) comprising such a cell.

Claims

1. An electrolyte composition comprising: a sulfonic acid of formula R-SO.sub.3H, in which R represents a (C.sub.1-C.sub.4)alkyl or a (C.sub.6-C.sub.14)aryl optionally substituted with a (C.sub.1-C.sub.4)alkyl, optionally sulfuric acid, redox metal ions, at least one inorganic additive (A) comprising at least one phosphorus atom whose oxidation state is less than or equal to +5, and water.

2. The composition according to claim 1, in which the sulfonic acid is chosen from the group consisting of: methanesulfonic acid, ethanesulfonic acid, benzenesulfonic acid, 1-naphthalenesulfonic acid, 2-naphthalenesulfonic acid and p-toluenesulfonic acid, preferably methanesulfonic acid.

3. The composition according to claim 1, comprising sulfuric acid, preferably comprising a mixture of methanesulfonic acid and sulfuric acid.

4. The composition according to claim 1, in which said inorganic additive (A) is chosen from the group consisting of: hypophosphorous acid, phosphorous acids, hypophosphoric acid, phosphoric acids, polyphosphoric acids, salts thereof and mixtures thereof.

5. The composition according to claim 1, in which said inorganic additive (A) is chosen from the group consisting of: hypophosphorous acid, metaphosphorous acid, pyrophosphorous acid, orthophosphorous acid, hypophosphoric acid, metaphosphoric acid, pyrophosphoric acid, orthophosphoric acid, triphosphoric acid, salts thereof, sodium hexametaphosphate and mixtures thereof.

6. The composition according to claim 1, in which the amount of inorganic additive(s) (A) is less than or equal to 5% by weight, preferably is comprised between 0.5% and 3% by weight, relative to the total weight of the electrolyte composition.

7. The composition according to claim 1, in which the redox metal ions are vanadium ions, preferably chosen from the group consisting of: V.sup.2+, V.sup.3+, VO.sup.2+, VO.sub.2+ and mixtures thereof.

8. The composition according to claim 1, also comprising a corrosion inhibitor.

9. The composition according to claim 8, in which the corrosion inhibitor is chosen from the compounds of general formula (1) or (2) below: NO2X (1) or NO3X (2) in which X is chosen from: Na; K; NH.sub.4; H; and when the corrosion inhibitor is a compound of formula (1), then X may also be chosen from: a linear or branched alkyl radical R′ comprising from 1 to 6 carbon atoms; an aryl radical Ar which is optionally substituted, in particular with at least one alkyl radical R′; a radical —SO.sub.2-G, where G represents H, OH, R′, OR′, OM, Ar, OAr, NH.sub.2, NHR′ and NR′R″, in which R′ and Ar are as defined previously, R″ represents a linear or branched alkyl radical comprising from 1 to 6 carbon atoms and M represents a monovalent or divalent metal cation, preferably an alkali metal or alkaline-earth metal cation; and a radical —CO-G, in which G is as defined previously.

10. An electrochemical cell including a negative electrode, a positive electrode and an electrolyte composition as defined in claim 1.

11. A redox battery comprising at least one electrochemical cell as claimed in claim 10, preferably a vanadium redox flow battery.

12. Use of an inorganic additive (A) comprising at least one phosphorus atom whose oxidation state is less than or equal to +5 for increasing the concentration of redox metal ions and/or avoiding or reducing and/or slowing down or delaying the precipitation of the redox metal ions in an electrolyte composition as defined in claim 1.

13. Use of an inorganic additive (A) comprising at least one phosphorus atom whose oxidation state is less than or equal to +5 for stabilizing an electrolyte composition as defined in claim 1 at a temperature of between 0° C. and 60° C., preferably between 5° C. and 50° C.

Description

EXAMPLES

Example 1: Stability of aqueous electrolytes for vanadium redox flow batteries comprising methanesulfonic acid (MSA) and one or more phosphorus additive(s) at high and/or low temperature

[0110] The thermal stability of electrolytes for a vanadium redox flow battery comprising a mixture H.sub.2SO.sub.4/MSA/inorganic phosphorus additive(s) was compared with that of a conventional vanadium redox flow battery electrolyte, such as those found commercially, for example those sold by the company Oxkem (https://www.oxkem.com/_html/product_pages/vanadium_sulfate_electrolyte.html) or by the company GfE (https://www.gfe.com/en/products-and-solutions/vanadium-chemicals/product-overview), for which:

[0111] the concentration of vanadium in oxidation state +4 (V+4) is generally about 1.55-1.75M (mol/l),

[0112] the concentration of sulfuric acid (H.sub.2SO.sub.4) is generally about 2-3M, and

[0113] the concentration of stabilizing additive, which is usually phosphoric acid, is about 0.05M.

[0114] A series of electrolytes from 99.9% vanadyl sulfate VOSO.sub.4, H.sub.2O.sub.4.8 (V4+) from the company Alfa Aesar, 95% sulfuric acid (H.sub.2SO.sub.4) from the company Carl-Roth, 99.5% methanesulfonic acid from the company Arkema and 85% phosphoric acid (H.sub.3PO.sub.4) (Ph.Eur p.a.) in water, from the company VWR, was prepared (see table 1 below). For this, the desired adequate amount of VOSO.sub.4 is weighed out and added to about 10 ml of water pre-acidified with the desired amount of acids (sulfuric and/or methanesulfonic and/or phosphoric) calculated for a final volume of 15 ml. The mixtures obtained are heated to 60° C. in a water bath to dissolve the vanadyl sulfate.

[0115] When dissolution is complete, the amount of water necessary to obtain 15 ml of electrolyte is added at 60° C. and allowed to cool to 20-23° C. After at least 2 days of stabilization, the concentrations of vanadium +4 and +5 are measured by cerimetric titration (see table 1 below):

TABLE-US-00001 TABLE 1 Composition of the vanadium (+4) electrolytes Electrolyte Electrolyte Electrolyte V + 4 V + 4 according to reference without the invention with V + 4 additive H.sub.3PO.sub.4 Molar concentration of 3M 2.75M 2.75M H.sub.2SO.sub.4 used for the dissolution of VOSO.sub.4 Molar concentration of 4.7M 4.45M 4.45M total sulfates Molar concentration of — 0.25M 0.25M MSA Molar concentration of 0.05M — 0.05M H.sub.3PO.sub.4 Relative molar 99.7% 100% 90.7% concentration of V + 4 Relative molar  0.3%  0%  0.3% concentration of V + 5 Total molar concentration of 1.7M 1.7M 1.7M vanadium

[0116] The three electrolytes prepared above were then electrolyzed in an electrochemical cell according to a conventional method in order to obtain electrolytes V+5 and V+3 for the thermal stability tests.

[0117] On conclusion of this electrolysis, two other additives were added to electrolytes V+3 and V+5 containing MSA (but no phosphoric acid): [0118] diammonium phosphate: 99.9% (NH.sub.4).sub.2PO.sub.4 from the company Sigma-Aldrich, [0119] a 50/50% mass mixture of 99.9% potassium phosphate K.sub.3PO.sub.4 and 96% sodium hexametaphosphate (NaPO.sub.3).sub.n from the company Sigma-Aldrich.

[0120] Finally, 1 ml of each electrolyte was placed in a small plastic tube and the samples were placed in an oven at 49-51° C. and visually inspected every day until the appearance of the first solid particles or the start of a color change, signs of electrolyte degradation. This determines the “induction time”, i.e. the stability time of the electrolyte at the temperature studied. The vanadium concentrations in the supernatant liquid are then determined to quantitatively estimate the proportion of vanadium that has precipitated.

[0121] The compositions and induction times of the various electrolytes subjected to the thermal stability tests are described in table 2 below:

TABLE-US-00002 TABLE 2 Induction time and composition of electrolytes V + 5 before/after thermal test at 50° C. Electrolyte 1 Electrolyte 2 Electrolyte 3 (V + 5) (V + 5) (V + 5) Electrolyte according to the according to the according to the V + 5 invention with invention with invention with reference H.sub.3PO.sub.4 K.sub.3PO.sub.4/(NaPO.sub.3).sub.n (NH.sub.4).sub.2PO.sub.4 Motor concentration — 0.25M 0.25M 0.25M of MSA Motor concentration 0.05M 0.05M — — of H.sub.3PO.sub.4 Mass concentration — — 1%/1% — of K.sub.3PO.sub.4/(NaPO.sub.3).sub.n Molar concentration — — — 0.1M of (NH.sub.4).sub.2PO.sub.4 Relative molar  0.7%  0.7%  0.9%  0.9% concentration of V + 4 before thermal test Relative molar 99.3% 99.3% 99.1% 99.1% concentration of V + 5 before thermal test Relative molar  1.5%  1.2%  1.3%  1.4% concentration of V + 4 after thermal test Relative molar 98.5% 98.8% 98.7% 98.6% concentration of V + 5 after thermal test Total molar 1.48M 1.64M 1.68M 1.70M concentration of vanadium after thermal test Induction time (days) 5 6 18 14

[0122] The results in table 2 clearly show that the electrolytes according to the invention make it possible to significantly improve the stability of the vanadium-based electrolyte since, firstly, the total vanadium concentrations after the thermal test are not very different, or are even equal to the initial concentrations before the test (1.7 M), unlike the reference electrolyte.

[0123] Secondly, the first signs of degradation of the electrolyte appear later than for the reference electrolyte, even up to 13 days longer for electrolyte 2 according to the invention.

[0124] Moreover, the compositions H.sub.2SO.sub.4/MSA/Additives according to the invention also allow good stability at low temperature. It is known that the V+3 and V+2 electrolytes are the most sensitive to low temperatures. However, none of the V+3 electrolyte solutions obtained after electrolysis of the V+4 solutions showed any sign of degradation (change in color or appearance of solid particles) after 8 days at 5° C.

[0125] In summary, the electrolyte compositions according to the invention show excellent thermal stability, in particular for vanadium redox flow batteries.